Recently, headlamps producing a continuously variable illumination range have become available. The illumination range may be varied by one or both of changing the intensity of light and changing the direction of light emitted by the headlamps.
Varying headlamp illumination intensity can be accomplished in several different means. A first means is to provide a pulse width modulated (PWM) signal to the headlamp. By varying the duty cycle of headlamp power, the headlamp illumination intensity can be increased or decreased. This may be accomplished by providing a PWM signal from a control system to a high power field effect transistor (FET) in series with the headlamp bulb.
Another means of varying the power duty cycle of a headlamp is to provide a PWM signal to a lamp driver integrated circuit such as a Motorola MC33286. This integrated circuit provides the added advantage of limiting the maximum inrush current to the headlamp, thus potentially extending the life of the headlamp bulb.
Yet another means of varying headlamp illumination uses high intensity discharge (HID) headlamps. HID lamps are a new, highly efficient headlamp technology. The ballasts used to power HID headlamps can be directly supplied with a control signal to vary headlamp illumination intensity.
Still another means to vary the illumination intensity of a headlamp is to provide an attenuating filter to absorb some of the light emitted from the headlamp. An electrochromic filter may be placed in front of the headlamp. By controlling the voltage applied to the electrochromic filter, the amount of light absorbed and, hence, the emitted illumination level, can be varied.
There are also several means available for changing the direction of light emitted from headlamps. Headlamp aim can be varied using actuators to move the headlamp housing relative to the vehicle. Typically, these actuators are electric motors such as stepper motors.
For headlamps with appropriately designed reflectors, mechanically moving the light source relative to the reflector can change headlamp beam direction as well as headlamp illumination intensity.
HID headlamps provide several additional methods of aiming the headlamp beam. Some of these methods involve deflecting or perturbing the arc in such away as to vary the lamp output. U.S. Pat. No. 5,508,592 entitled "Method For Deflecting The Arc Of An Electrodeless HID Lamp" to W. Lapatovich, S. Butler, J. Bochinski, and H. Goss, which is hereby incorporated by reference, describes exciting the HID lamp with a high-frequency radio signal. Modulating the signal causes the lamp to operate at an acoustic resonance point, perturbing the arc from its quiescent position. An alternative technique, known as magnetodynamic positioning (MDP), uses a magnetic field to shape the HID arc. MDP is being developed by Osram Sylvania Inc. of Danvers, Massachusetts.
A collection of methods for implementing continuously variable headlamps is described in Society of Automotive Engineers (SAE) publication SP-1323 entitled "Automotive Lighting Technology", which is hereby incorporated by reference.
Automatic control of continuously variable headlamps offers several potential benefits over automatic control of traditional on-off headlamps. Greater flexibility for illumination is available, allowing headlamp illumination to be better adapted to driving conditions. Also, continuously varying the headlamp illumination does not create rapid changes in illumination that may startle the driver. Various methods have been devised to control both continuously variable and conventional discreet headlamps. One of the oldest methods is to aim the headlamp in the same direction as steered wheels. Another method increases the illumination range in proportion to increasing vehicle speed.
Still another method of controlling headlamps has been developed for HID lamps. The increased brightness and blueish color of the HID lamps is particularly disrupting to oncoming drivers. Due to this disruptness effect, certain European countries require headlamp leveling systems if HID lamps are used on a vehicle. These headlamp leveling systems detect the pitch of the vehicle relative to the road and adjust the vertical aim of the headlamps accordingly. Advanced systems further use the speed of the vehicle to anticipate small pitch disturbances caused by acceleration.
One problem with current continuously variable headlamp control systems is the inability to consider oncoming or leading vehicles in determining the illumination range of headlamps. One prior art device is expressed in U.S. Pat. No. 4,967,319 entitled "Headlight Apparatus For Automotive Vehicle" by Y. Seko. This device utilizes vehicle speed along with the output from a five-element linear optical sensor array directly coupled to a headlamp. The headlamp incorporates motor drives to adjust the elevational angle of illumination beams. This design requires separate sensing and control system for each headlamp or suggests as an alternative a controlled headlamp only on the side of the vehicle facing opposing traffic. This design presents many problems. First, the optical sensor and associated electronics are in close proximity to the hot headlamp. Second, placing the image sensor on the lower front portion of the vehicle may result in imaging surfaces being coated with dirt and debris. Third, placing the image sensor close to the headlamp beam makes the system subject to the masking effects of scattered light from fog, snow, rain, or dust particles in the air. Fourth, this system has no color discriminating capability and, with only five pixels of resolution, the imaging system is incapable of accurately determining lateral and elevational locations of headlamps or tail lights at any distance.
What is needed is control of continuously variable headlamps based on detection of oncoming headlamps and leading tail lights at distances where headlamp illumination would create excessive glare for the drivers of oncoming and leading vehicles.